improve linux terminal link error message

[bebehei]

After clicking on the exec-Button, a message popped up, stating The terminal emulator symbolic link doesn\'t exists. Please read the Wiki at https://github.com/docker/kitematic/wiki/Early-Linux-Support..

Sadly:

1. the message gives no indication of the problem.
2. The link given in the message, points to the wiki-site Early-Linux-Support, which is except a link is empty
3. The Link contains an anchor #early-linux-support-from-zedtux, which does not exist on the linked Wiki-Site.

I got my indication out of #1648.

It may also be good to remove the bad links in the message (and) remove the bad wiki-pages.

[FrenchBen]

Thanks for fixing this - Would you be able to provide an alternative terminal emulator that worked for you?

[bebehei]

Using the symlink /usr/bin/x-terminal-emulator is the perfect thing. It gives programmers the ability to use any terminal-emulator without big hassle.

It's just not linked on every system. This PR only improves the error message, nothing more. The next user should know, what to fix.

[FrenchBen]

It seems that the wiki should then be updated, as the page you mention: https://github.com/docker/kitematic/wiki/Early-Linux-Support points to another wiki page, which has none of the Linux information.

Do you mind updating that page also noting what distro needs the symlink created?

[bebehei]

@FrenchBen I changed the corresponding wiki page. Could you please merge it?

[FrenchBen]

Thanks for the patch!